This invention relates to sensing the impedance of conductors disposed to span expected cracks in and near the surface of fracture-critical machinery components.
In gas turbine jet engines, and other rotating machinery, the designation xe2x80x9cfracture-criticalxe2x80x9d is given to those components with enough energy, due to high rotational speeds and mass, to promote catastrophic failure. For example, in gas turbine engines, fan, compressor and turbine disks are fracture-critical. Providing containment for this type of failure is unrealistic and it is not a standard practice. Instead, fractures in the rotating components of gas turbine engines are avoided through adequate design and quality control margins, and through periodic inspections. The cost for disassembly, inspection and reassembly over the life of an engine is on the order of the initial cost of the engine.
An alternative approach to managing failures in rotating parts is use of a sensor capable of detecting impending failure with sufficient advance warning to allow timely maintenance action. A sensor that induces eddy currents in a gas turbine disk is disclosed in Barranger, J. P., xe2x80x9cEddy Current Jet Engine Disk-Crack Monitorxe2x80x9d, Materials Evaluation, Vol. 42, October 1984, pp. 1374-1378. The sensor is disposed on a stationary part adjacent the rotating part, and the combined capacitance and conductance of the device is dependent upon the integrity of the disk. Changes in electrical properties of the device are correlated with cracks in the disk.
Optical fibers have been used to monitor a condition directly, visually, and to measure the sources of structural wear, such as stress, strain and so forth, using wear indicating models to infer the actual condition. However, this technology by its very nature is hard to apply in a rotational environment such as engine disks.
The diagnostic signals of the prior art gradually degrade with time, can be analyzed only using long-term histories; can only detect cracks that actually move past the sensor; and cannot detect cracks which are remote from the sensor or hidden on the rotating part.
Objects of the invention include crack detection on machinery, including particularly rotating components: which does not require memory and long-term history; in which the diagnostic signals do not gradually degrade with time; substantially anywhere in the rotating structure; which is essentially instantaneous; in which the degree of crack progression is sensed quantumly; wherein cracks may be detected in rotating parts in which both vibratory and centrifugal forces are extremely large; wherein temperatures in the sensor and environment are extremely high; which is readily implemented using processes and materials known to have satisfactory historical usage, with low failure rates and high reliability.
According to the present invention, one or more circuits, each including one wire or several wires electrically connected in parallel with each other, are each connected between an excitation node on one end and a detection node on the other end, creating excitable closed circuits. In one embodiment, the excitation and detection nodes are inductive.
According further to the invention, the wires, excitation nodes and detection nodes are coated on the machine part by a suitable process, such as vapor deposition. The configuration of the wiring is effected by etching, utilizing techniques which are well known in the fabrication of semiconductor integrated circuits.
In further accord with the invention, materials the same as and/or compatible with the material of the machine part may be used to form the wires, excitation nodes and detection nodes.
The invention permits sensing the development of cracks in and near the surface of rotating and other parts, which cracks may be located remotely from the sensor itself, and even in hidden areas (such as on a back surface of the part). By causing the loss of one wire commensurately with each incremental length of a crack, the invention provides a quantum change in the impedance of the circuit, which is easy to process and present substantially instantaneous indications thereof.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.